1. Field of the Invention
The invention is generally related to risers used in the production of hydrocarbons and more particularly to the use of production risers and associated equipment in relation to handling slugs, riser base gas lift and metering flows.
2. General Background
In the production of oil and gas, slugging can occur in multiphase production flowlines as a result of any of the following mechanisms: hydrodynamic slugging; terrain induced slugging; production flow rate changes; or severe slugging at the base of the riser. Slugging is the phenomenon of fluids flow instability.
Hydrodynamic slugging is caused by the nature of the physical property differences between the highly compressible gas phase and relatively incompressible liquid phase being transported together in a flowline. The nature, size, and frequency of slugs are statistically dependent and, therefore, 99-percentile slug size is used to size slug catchers, for example.
Terrain induced slugging occurs when liquid is held up in the dips of the flowline due to reduced production rates and the profile of the terrain. Also, severe slugging can result upon a production restart or rate increase.
Production flow rate changes can increase the liquid hold up in the flowline when production flow rate is reduced, which is then swept out as a large slug upon flow rate increase.
Severe slugging can be considered as a special case of terrain induced slugging and occurs when the flowline inclines downwards before the vertical riser and the flow regime is segregated.
Conventional systems use large and costly slug catchers on the receiving facilities to mitigate slugs, stabilize flow, and improve separation efficiency. The space on the receiving facility (offshore platform) is tight and at a premium, along with operational problems such as control trips resulting from slugs and gas surges that cause loss of production and hence revenue.
A riser based gas lift system is used to complement slug catchers to mitigate slugs, especially during systems start up. Riser based gas lift systems are generally complicated and costly. They require an unwanted degree of systems complexity, both at the subsea location (e.g., diffusers) and topsides on the platform (e.g., chemical inhibition system, valving, heating, power, etc.).
Existing multiphase meters are highly complex devices that use sophisticated mechanisms such as gamma radiation and mixers, as well as power and data capture and transmission equipment, all of which limit accuracy (+/-10%), reliability, and increase costs.
It can be seen that the current state of the art leaves a need for equipment that is capable of handling slugging with reduced cost and complexity.